In two fluid problems the density is computed from the fluid fraction, not the other way around. If you define fluid 1 as water and fluid 2 as oil then you just specify the mixture fluid fraction in the initial conditions and at the boundaries. For example, if you had a mixture of 5% oil and 95% water entering at a boundary you would specify a fluid fraction of 0.95 at this location.

if i choose two number of fluid then there is no free surface (no out flow) in my case i have a free surface separator tank and the inlet flow is mixture between oil and water.
which the best model must be use
Drift flux
source mass
or
scalars

Sorry - I didn't realize that there was a free surface. In this case you'll probably want to use the 1 fluid drift flux model and specify the mixture density at the boundaries, initial conditions, and sources.